Backwash

ABSTRACT

A method of backwashing a membrane filtration module ( 4 ), said module ( 4 ) including one or more membranes ( 5 ) located in a feed-containing vessel ( 3 ), the membranes ( 5 ) having a permeable wall which is subjected to a filtration operation wherein feed containing contaminant matter is applied to one side of the membrane wall and filtrate is withdrawn from the other side of the membrane wall, the method including: removing liquid from the feed-containing vessel ( 3 ) until the level of liquid in the feed-containing vessel ( 3 ) falls to a first level (L 2 ) below an upper level of the membranes; suspending the filtration operation; performing a liquid backwash of the membrane wall until liquid level within the feed-containing vessel ( 3 ) rises to a second predetermined level (L 1 ) above said first level; aerating the membrane surface with gas bubbles to dislodge fouling materials therefrom; performing a sweep or drain down of the feed-containing vessel ( 3 ) to remove the liquid containing the dislodged contaminant matter; and recommencing the filtration operation.

TECHNICAL FIELD

The present invention relates to membrane filtration systems and moreparticularly, the backwashing of such systems.

BACKGROUND OF THE INVENTION

Porous membrane filtration systems require regular backwashing of themembranes to maintain filtration efficiency and flux while reducingtransmembrane pressure (TMP) which rises as the membrane pores becomeclogged with impurities. Such systems usually have a number of membranesin the form of bundles or arrays of membranes located in a feed tank.Feed is thus applied to the surfaces of the membranes and filtratewithdrawn from the membrane lumens. Typically, during the backwash cyclethe impurities are forced out of the membrane pores by pressurised gas,liquid or both into the feed tank or cell. The liquid containingimpurities and deposits from the membranes is then drained or flushedfrom the tank.

The waste liquid displaced from the tank needs to be disposed of orreprocessed, usually in an environmentally safe manner, so any reductionin the volume of such waste liquid is seen as advantageous in terms ofenvironmental impact and cost.

The draining or flushing of the tank, particularly when large arrays ofmembranes are used also requires time which results in down time of thefiltration cycle. In order to reduce this down time large pumpingsystems are required to quickly drain and refill the tank. Where tanksor cells are arranged in banks and feed is used to refill the tank, alowering in levels in other cells may be produced during the refillprocess. This again impinges on operating efficiency of the filtrationsystem.

Reduction in backwash volume also reduces the volume of chemicalcleaning agents required in some systems. This has the two-foldadvantage of reducing cost in terms of chemical requirements while alsoreducing waste disposal problems.

Many filtration systems also employ a gas bubble aeration method toassist in scouring unwanted deposits from the surfaces of the porousmembranes during the cleaning stage. In order for such aeration to beeffective the membranes must be fully immersed in liquid.

Typically the cleaning p in such systems includes ceasing flow of feedliquid to the vessel in which the membranes are located, continuing thefiltration process until the level of liquid in the tank reaches the topof the membranes, aerating the membranes with gas bubbles to scourimpurities from the outer membrane surfaces for a period of time, thenliquid backwashing the membrane pores. The tank is then drained orflushed to remove the waste liquid containing the impurities dislodgedduring the aeration and backwashing processes.

It will be appreciated that it is necessary to keep the membranessubmerged so as to make the aeration step effective. Adding the liquidbackwash at the end of the aeration step results in the liquid level inthe tank rising above the membranes. This is “wasted volume” in terms ofthe requirement of keeping the membranes submerged.

SUMMARY OF THE INVENTION

The present invention seeks to overcome or at least ameliorate this andother disadvantages of the prior art.

According to one aspect, the present invention provides a method ofbackwashing a membrane filtration module, said module including one ormore membranes located in a feed-containing vessel, the membranes havinga permeable wall which is subjected to a filtration operation whereinfeed containing contaminant matter is applied to one side of themembrane wall and filtrate is withdrawn from the other side of themembrane wall, the method including:

-   -   a) removing liquid from the feed-containing vessel until the        level of liquid in the feed-containing vessel falls to a first        level below an upper level of the membranes;    -   b) suspending the filtration operation;    -   c) performing a liquid backwash of the membrane wall until        liquid level within the feed-containing vessel rises to a second        predetermined level above said first level;    -   d) aerating the membrane surface with gas bubbles to dislodge        fouling materials therefrom;    -   e) performing a sweep or drain down of the feed-containing        vessel to remove the liquid containing the dislodged contaminant        matter; and    -   f) recommencing the filtration operation.

Preferably, the second predetermined level is selected such that theliquid level rises at least to the upper level of the membranes duringthe aeration step. Preferably, the flow of feed into the feed-containingvessel is suspended prior to removal step a). For preference, theremoval of liquid is achieved fully or partially by the filtrationprocess. Preferably, in a pressurized filtration system, a pressurizedgas is employed during the filtration process to push the liquid fromthe feed side through the permeable membrane wall during the removalstep a). The aeration step may commence prior to removal of liquid stepand continue until the sweep or drain down step e).

This process has the advantage of reducing the backwash volume. Also,rather than filtering down to the required liquid level, it is possibleto drain or pump away liquid in the feed-containing tank to achieve thesame result. This has the advantage of not increasing the flux of themembrane during the filter down step, and can be done quickly to reducefiltration down time. The drained feed may be returned to the inlet ofthe filtration plant so as not to be wasted, or transferred to othertanks operating in parallel.

In an alternate method, the filtration step may be done at varyingfiltrate flows; the flow being adjusted to maintain a constanttransmembrane pressure (TMP) as the liquid level drops down themembranes.

The TMP may be monitored during the removal of liquid step and used todetermine when the liquid level has dropped below the top of themembranes, or how far below the upper level of the membranes the liquidlevel has dropped. This is possible because as more of the membrane isexposed the filtration area is less and the TMP will increase. TMP willalso increase due to the fact that more of the filtrate flow travelsfurther along the lumen so increasing the lumen pressure drop (assumingthe flow is kept constant and filtrate is being withdrawn from the upperend of the membrane lumen). Accordingly, the change in TMP may be usedto monitor the liquid level in the feed tank before commencing theliquid backwash step. This may also be used to adjust the filtrate flowand maintain a constant TMP so avoiding significant changes in the fluxon the remaining filtrate area. Alternatively, such a method can be usedto control to the TMP to any desired predetermined level, for example,it may desirable to lower the liquid level in the feed-containing tank,keeping the TMP at the maximum allowable TMP, say 85 kPa.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIGS. 1 a to 1 e show schematic views of the membrane module at variousstages of the backwash process according to an embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the FIGS. 1 a to 1 e, an illustrative embodiment of theinvention is described. The filtration module 4 is mounted within ahousing vessel 3 which contains the feed to be filtered. The filtrationmodule 8 contains a bundle or bundles of hollow fibre membranes 5extending between upper and lower headers 6 and 7, respectively. Thelower header 7 is provided with a number of openings 9 communicatingwith the interior of the fibre bundle and an inlet port 10. Feed issupplied through port 11 under the control of valve AV1.

Permeate/filtrate is withdrawn through chamber 12 and line 13. A liquidbackwash may also be applied through line 13.

FIG. 1 a shows the module in normal filtration operation with the liquidlevel in the vessel 3 at L1 covering the upper header 6. When thebackwash stage is to be commenced, in this embodiment, the flow of feedis stopped by shutting valve AV1. Filtrate continues to be withdrawnthrough the fibre lumens and chamber 12 and line 13. The level of liquidin the vessel 3 falls, as shown in FIG. 1 b, to level L2.

Once level L2 is reached, a liquid backwash is commenced by flowingliquid back through the fibre lumens and walls via line 13. This resultsin the liquid level again rising in the vessel to L1 as shown in FIG. 1c. An aeration step is then commenced by feeding gas through port 10 andopenings 9 as shown in FIG. 1 d. In another embodiment, the liquid levelresulting from the liquid backwash may be just below level L1 such thatlevel L1 is reached during the aeration step with the liquid levelrising further as a result of bubbles present in the liquid. A similarresult may be achieved by commencing the aeration step during the liquidbackwash.

Once the aeration step is completed the vessel is swept or drained asshown in FIG. 1 e to remove dislodged waste materials accumulated in thevessel liquid. Following this step feed is reintroduced to the vessel byopening valve AV1 and filtration recommenced.

Although the embodiment relates to vertically orientated membranes, itwill be appreciated that the invention is not limited to such anorientation and the membranes may be orientated in any desired positionor configuration.

It will be appreciated that the invention may be applied to bothpressurized and non-pressurized systems where the feed-containing vesselis open to atmosphere.

It will be appreciated that further embodiments and exemplifications ofthe invention are possible with departing from the spirit or scope ofthe invention described.

1. A method of backwashing one or more membranes in a vessel, the method comprising: removing liquid to be treated from the vessel until a level of the liquid falls to a first level within the vessel below an upper level of the one or more membranes, wherein the removal of liquid to be treated is at least partially accomplished by a filtration operation; suspending the removal of the liquid to be treated from the vessel when the level of the liquid to be treated falls to the first level; performing a liquid backwash of the one or more membranes when the first level is reached by the removal of the liquid to be treated from the vessel until the level of the liquid collected in the vessel comprising backwashed liquid is at a predetermined level above said first level; aerating a membrane surface of the one or more membranes; and removing liquid containing fouling materials dislodged from the membrane surface from the vessel; wherein the predetermined level provides sufficient liquid in the vessel to immerse an upper header of the one or more membranes during the aerating of the membrane surface.
 2. The method according to claim 1, wherein the step of aerating the membrane surface comprises aerating the one or more membranes until a level of liquid comprising backwashed liquid within the vessel rises to at least the upper level of the one or more membranes.
 3. The method according to claim 1, further comprising a step of suspending a flow of the liquid to be treated into the vessel prior to performing the step of removing liquid to be treated.
 4. The method according to claim 1, wherein the step of aerating the membrane surface is commenced before performing the step of removing liquid to be treated, and is performed until the step of removing liquid containing fouling materials is performed.
 5. The method according to claim 1, further comprising a step of applying a pressurized gas to the liquid to be treated during the filtration operation.
 6. The method according to claim 1, wherein the step of removing liquid to be treated comprises draining the liquid from the vessel.
 7. The method according to claim 1, wherein the step of removing liquid to be treated comprises pumping the liquid from the vessel.
 8. The method according to claim 1, further comprising a step of returning liquid to be treated that was removed from the vessel back into the vessel.
 9. The method according to claim 1, further comprising a step of introducing into the vessel liquid to be treated which has been removed from a second vessel.
 10. The method according to claim 1, wherein the step of removing liquid to be treated comprises maintaining a substantially constant transmembrane pressure across the one or more membranes.
 11. The method according to claim 10, wherein the transmembrane pressure is maintained at or below a predetermined value.
 12. The method according to claim 1, further comprising a step of monitoring the transmembrane pressure across said one or more membranes.
 13. The method according to claim 12, wherein the step of monitoring the transmembrane pressure is performed while performing the step of removing liquid to be treated.
 14. The method according to claim 12, further comprising a step of determining the level of liquid in the vessel.
 15. The method of claim 1, wherein the predetermined level is the same as a level of liquid in the vessel prior to removing liquid to be treated.
 16. The method of claim 1, wherein aeration of the membrane surface is commenced while performing the liquid backwash.
 17. The method of claim 1, wherein the membranes are non-vertically oriented.
 18. The method of claim 1, wherein the predetermined level is below the upper level of the one or more membranes.
 19. A method of backwashing a membrane filtration module in a vessel, said module including one or more membranes, the method comprising: removing liquid from the vessel until a level of the liquid falls to a first level within the vessel below an upper level of the one or more membranes, wherein the removal of liquid from the vessel is at least partially accomplished by a filtration operation; suspending the removal of the liquid from the vessel when the level of the liquid falls to the first level; performing a liquid backwash of the one or more membranes, after removing the liquid from the vessel, until the level of liquid in the vessel rises to a predetermined level above the first level; initiating aeration of the membrane surfaces of the one or more membranes after the liquid backwash is completed; and removing liquid containing fouling materials dislodged from the membrane surfaces from the vessel, after aerating of the membrane surfaces is completed; wherein the acts of removing liquid from the vessel, performing a liquid backwash, aerating membrane surfaces, and removing liquid containing fouling materials are performed within a single cycle, and wherein the predetermined level provides sufficient liquid in the vessel to immerse an upper header of the membrane filtration module during the aeration of the membrane surfaces.
 20. The method of claim 19, wherein the first level is located above a lower header of the membrane module and below an upper header of the membrane module.
 21. The method of claim 19, wherein the predetermined level is below the upper level of the one or more membranes.
 22. The method of claim 19, wherein the step of performing a liquid backwash introduces less fluid into the vessel than the step of removing liquid from the vessel removes.
 23. A method of treating water, comprising: introducing water to be treated into a vessel; filtering the water to be treated through membranes immersed in the water; reducing a level of the water to be treated to a first level within the vessel below an upper level of the membranes, wherein the reduction of the level of the water to be treated is at least partially accomplished by a filtration operation; commencing a backwash of the membranes when the first level is reached by the reduction of the level of water to be treated, the backwash continuing until a level of a water solution comprising backwashed liquid in the vessel rises to a second predetermined level above the first level; and aerating the membranes; wherein the second predetermined level provides sufficient liquid in the vessel to immerse an upper header of the membranes during the aeration of the membranes.
 24. The method according to claim 23, further comprising a step of removing the water solution in the vessel after aerating the membranes. 